The determination of gas and liquid flow rates in gas-liquid fluid mixtures is important for monitoring of rotodynamic pumps. Rotodynamic pumps often have a recommended operating range with respect to flow rate. If the flow rate is too high or too low (surge occurrence), the pump lifetime may be reduced and the risk of mechanical failure is increased. This is true for both single-phase and multiphase rotodynamic pumps.
For a single-phase pump, a “single phase” flowmeter (venturi, V-cone, orifice plate or similar) may be utilized to monitor the flow rates seen by the pump. These flow meters have a differential pressure transmitter, and based on the fluid density the flow rate can be calculated. The calculated flow rate is used to indicate whether the pump is being operated in a safe range. If not, the pump will “trip” or the control system will adjust speed, recycling rate or other parameters to move the operating point inside the recommended operating range.
For multiphase pump systems, a difficulty is that the mixture density varies with time and is generally not known. As a result, the flow meters used for a single phase pump are not accurate since the flow rate cannot accurately be calculated when mixture density is unknown.
In some cases, historical data for multiphase fluids is used for multiphase flow rate calculations. In the example of pumps used within the oilfield services applications, watercut and gas oil ratio (GOR) can be used as manual input. A minimum flow rate can then be calculated based on pressure, temperature and speed measurements. However, this method can suffer from poor reliability and inaccurate results, particularly since the actual watercut or GOR vary with time and thus are likely different from historical data used by the control system.
Another solution to protect a multiphase pump from undesirable high or low flow in the oilfield domain is to use an apparatus like the Schlumberger's VX™ system, which comprises a vertically mounted Venturi flow meter, a dual energy gamma-ray hold up measuring device and associated processors. This system allows the simultaneous calculation of gas, water and oil volumetric flow rates in multiphase flows. However, despite providing proven performance, the VX™ system and other conventional multiphase flow meters are relatively expensive and complex. Their reliability/availability is often lower than standard “single phase” meters. Another drawback is that multiphase meter measurements are based on statistics and a 30-60 second sampling time is often used. As a result, the method may be less able to detect rapid changes in fluid densities (e.g., gas/liquid slugs).